As one of those who tie down, I use a Bruce's canopy cover for protection. Over the years a large black vapor stain has appeared over the vented fuel cap. It looks like black mold. I contacted Bruce's about it and they say they have never heard of it nor received a comment/complaint about it.

For those who use a cover, have you had/seen a similar stain? I cannot believe I am the only one in the fleet to experience this. Thanks.

I only use Costco Avgas. And don't recall it being filled to a point where it ever overflows.

May not have to overflow to cause your problem. In fact, partially filled tank may be more of culprit. As temperature changes, the air in fuel tank expands and contracts causing fuel vapor to exit the breather hole in the cap.

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